Technical Field
The disclosure relates generally to fuel cells, and more particularly to fuel cell systems, accumulators therefor, and related methods. More particularly still, the disclosure relates to such fuel cell systems and accumulators for operation in sub-freezing temperatures.
Description of the Related Art
Fuel cell systems, such as fuel cell power plants that provide power to the propulsion system of electric vehicles, must be operable at temperatures below that at which water will freeze. Fuel cell systems using proton exchange membranes (PEM) are most typically utilized in such applications, and require a well-hydrated membrane for good operation and durability. During normal operation, water may be drawn through a PEM fuel cell from the anode to the cathode. Notably, water is also produced at the cathode. Traditional methods of operating fuel cells that are shut down in environments which may reach freezing temperatures involve draining some or all of the water out of the fuel cell into a reservoir of some sort, as for example an accumulator. Before trying to establish subsequent operation, frozen water must be melted before it can be moved back into the fuel cell, which process may take at least several minutes, resulting in a delay that is likely to be viewed as intolerable for such transportation usage, where delays preferably do not exceed a few seconds. Moreover, the melting of the frozen water typically requires the application of a source of energy that may adversely impact the efficiency of the system.
One approach to deal with a facet of these concerns is described in PCT International Published Application WO 2006/112833 A1, wherein the cells include porous water transport plates, and water contained only in the water channels, ducts or pores of cells during operation is retained there by capillary action and/or by vacuum during shutdown, and serves to provide some humidification and cooling at start-up. While this may at least partly address some problems, it relies upon the fuel cells having either coolant ducts of small size for the capillary action or the use of a vacuum, or both, to prevent water from “slumping” into the reactant channels of the fuel cells. Additionally, it allows some amount of freezing of the water within the cells themselves during shutdown, so long as it is limited to the coolant ducts and not the reactant channels. While this may be tolerable, it is less than desirable because it increases the time needed to bring the cell to operating temperature. Moreover, it does not really address the need to relatively rapidly and efficiently provide water from a reservoir, such as an accumulator, to the fuel cells at, or shortly after, start-up, under freezing conditions.